This is one of those things that's bugged me for years but I never remember to ask about it. I was tapping a hole this afternoon and realized that the arrangement for holding a tap in the handle has always struck me as unusual. For those who haven't seen it here are some pictures:

Each tap has a square end and it's secured into the handle by placing it into the 'V' shaped center and then tightening the screw. Why is it done this way with the screw thread pressing down on the edge of the tap? Hex keys have been made for decades and it would be a far more efficient way of holding the tap in place.

Attempting to clamp on the flats does not transfer the energy to the tap as well as the method of clamping the corners. This is not likely to be of much concern for the smaller taps and dies as used in modern electronics, but the design stems from a day when taps were rather large in size, men were men and the women were glad of it.

Clamping the corners also has to do with the fact that taps and dies are hardened. Those who have attempted to press an open end wrench or crescent wrench into service as a tap wrench likely find out real quick about clamping at the flats, especially if they are threading stainless stell or other hard metals.

Great-great-grandpa wasn't stupid. Things were done for a reason. Sometimes that reason gets clouded as time marches on, it is important for us not to fall prey to the idea that those who came before us somehow don't measure up to the wonderful amount of knowledge that we think we have today.

Give the old school the benefit of the doubt and things start to look a lot different.

All my taps have square ends on the shanks that vary with the size of the tap so there isn't one standard size.

If you clamp the tap shank across the corners in a "V" it easily holds it at a right angle to the handles (assuming the "v" is machined at a right angle) in two directions which would be more difficult if you tried to clamp across the flats and still allow for different shank sizes.

My smaller tap handles have a chuck something like a drill chuck with four fingers that do clamp across the flats but the range of tap sizes any one handle can accommodate is limited.

I also have a tap holder that has two blocks with opposing 'V's machined in them: one of them is movable by a screw thread. So there is a minimum tap size it will take and hold, but that holds on all four sides.

I also have a 6BA thread tap that only has two flats on it.....

And the most unusual of all is 5/16 BSW tap that is a left hand thread! No die to go with, though.

There is one other thing. In some places and for larger taps, sometimes the tap holder just can't fit properly depending on its construction. In cases like that, a small adjustable wrench is used by some to turn the tap--either in or out. That's possible with a six sided (hex) but it isn't quite as good as a four sided grip that is generally used on taps.

This is my first attempt to reply so I hope I get it right. I was a master tool and die maker in heavy industry for 45 years. The square in taps was and is used as the drive torque device for use in machine tools where drilling and tapping are done at high speed. The body of a tap is held in alignment by a holder with a precision hole that just fits the body of the tap. The clamping to hold the tap is done on the square. The machine has a system that drives the tap forward at exactly the thread lead so there is no in and out pressure on the tap only torque to cut the material being threaded. There are many variations in the design of a tap holder, but the tap handles used by hand are an after thought to the design of the tap.

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